On the impact of adsorbed gas molecules on the anisotropic electro-optical properties of β12-borophene

We theoretically study the role of adsorbed gas molecules on the electronic and optical properties of monolayer β12-borophene with {a,b,c,d,e} atoms in its unit cell. We focus our attention on molecules NH3, NO, NO2, and CO, which provide additional states permitted by the host electrons. Utilizing the six-band tight-binding model based on an inversion symmetry (between {a,e} and {b,d} atoms) and the Kubo formalism, we survey the anisotropic electronic dispersion and the optical multi-interband spectrum produced by molecule-boron coupling. We consider the highest possibilities for the position of molecules on the boron atoms. For molecules on {a,e} atoms, the inherent metallic phase of β12-borophene becomes electron-doped semiconducting, while for molecules on {b,d} and c atoms, the metallic phase remains unchanged. For molecules on {a,e} and {b,d} atoms, we observe a redshift (blueshift) optical spectrum for longitudinal/transverse (Hall) component, while for molecules on c atoms, we find a redshift (blueshift) optical spectrum for longitudinal (transverse/Hall) component. We expect that this study provides useful information for engineering field-effect transistor-based gas sensors.

Theoretical prediction of Janus PdXO (X = S, Se, Te) monolayers: structural, electronic, and transport properties

Due to the broken vertical symmetry, the Janus material possesses many extraordinary physico-chemical and mechanical properties that cannot be found in original symmetric materials.

Tuning electronic phase in noncentrosymmetric quantum spin Hall insulators through physical stimuli

In this work, the perturbation-induced phase transitions in noncentrosymmetric quantum spin Hall insulators (QSHIs) are analytically addressed. In particular, the dilute charged impurity, the electric field, and the Zeeman splitting field are considered within the tight-binding Hamiltonian model, Green's function approach, and the Born approximation. Following theC_3vsymmetry breaking in the PbBiI compound as a representative QSHI, the band gap becomes larger via the electric field, while the system transits to the semimetallic phase via the dilute charged impurities and Zeeman field, modifying the degenerate states in the electronic density of states. While the coexistence of electric field and impurities demonstrate that the system backs to its initial semiconducting phase, the combined Zeeman field and impurities do not alter the robust semimetallic phase.

Electronic, optical, and thermoelectric properties of Janus In-based monochalcogenides

Inspired by the successfully experimental synthesis of Janus structures recently, we systematically study the electronic, optical, and electronic transport properties of Janus monolayers In₂XY(X/Y= S, Se, Te withX≠Y) in the presence of a biaxial strain and electric field using density functional theory. Monolayers In₂XYare dynamically and thermally stable at room temperature. At equilibrium, both In₂STe and In₂SeTe are direct semiconductors while In₂SSe exhibits an indirect semiconducting behavior. The strain significantly alters the electronic structure of In₂XYand their photocatalytic activity. Besides, the indirect-direct gap transitions can be found due to applied strain. The effect of the electric field on optical properties of In₂XYis negligible. Meanwhile, the optical absorbance intensity of the Janus In₂XYmonolayers is remarkably increased by compressive strain. Also, In₂XYmonolayers exhibit very low lattice thermal conductivities resulting in a high figure of meritZT, which makes them potential candidates for room-temperature thermoelectric materials.

Electric field and charged impurity doping effects on the Schottky anomaly of β12-borophene

Due to the coexistence of Dirac and triplet fermions, monolayer β12-borophene has recently attracted both experimental and theoretical researchers. In particular, various phase transitions have been recently reported in the structure, in the presence of dilute charged impurity and a perpendicular electric field, leading to interesting electronic heat capacity (HC). In this paper, we systematically examine the effects of charged impurity doping and electric field on the HC of monolayer β12-borophene. To do this, we utilize the five-band tight-binding Hamiltonian model, the Green's function, T-matrix, and the Born approximation for different models considering the substrate effects. Numerical analysis reveals that the inversion symmetric model is the proper model in the pristine and perturbed metallic β12-borophene, leading to a regular reduction of HC with both charged impurity and electric field. Moreover, the pristine and perturbed Schottky anomaly alterations are fully addressed. Unforeseeably, HC irregularly fluctuates with impurity in the homogeneous model. We believe that our results provide new physical insights into the thermal properties of monolayer β12-borophene.

Impurity scattering effects on the validity of Fermi liquid theory in topological crystalline insulator SnTe (001) thin films

We study the electronic heat capacity (EHC) and the Pauli spin paramagnetic susceptibility (PSPS) of topological crystalline insulator SnTe (001) thin film in the presence of dilute charged impurities in an effective Hamiltonian model for the low-energy regime such as impurity concentration and impurity scattering potential effects. To calculate the EHC and PSPS using the Boltzmann method, we first calculate the electronic density of states by means of the Green's function approach. Also, the impurity effects are considered with the aid of the T-matrix approximation. We demonstrate that the hybridization potential between the front and back surfaces in SnTe (001) thin films leads to the band gap opening and to the zero PSPS at low temperatures obeying the Fermi liquid theory. In particular, we demonstrate two scenarios including the possibilities of the same and different impurity doping. It is shown that in both cases the midgap states emerge, the cation-anion symmetry breaks down and the Fermi liquid theory loses its validity. Moreover, the critical scattering potential with respect to the hybridization potential is found for the validity limit of the Fermi liquid theory. Finally, the Schottky anomaly and the crossover in EHC and PSPS, respectively, are discussed. Our results have strong implications for applications based on SnTe (001) thin films.

Stark and Zeeman effects on the topological phase and transport properties of topological crystalline insulator thin films

The fundamental investigation of topological crystalline insulator (TCI) thin films is essential for observing interesting phenomena. In practice, a promising pathway involves the application of electric and magnetic fields to tune the topological phases of TCI thin films. To achieve this, we applied a perpendicular electric field and an in-plane magnetic field to not only tune the Dirac gap of a SnTe(001) thin film and find the phase transition but also to directly connect them with their effects on the group velocity of both massless and massive surface Dirac fermions. The TCI thin film is an inherent insulator due to the hybridization between the front and back surfaces, and it transitions to a semimetal phase at a critical perpendicular electric field due to the Stark effect. Correspondingly, the anisotropic group velocity of the upper (lower) conduction (valence) band decreases (increases) with the electric field at certain momenta. We found that when one of the in-plane Zeeman field components becomes stronger than the intrinsic hybridization potential, the anisotropic Weyl cones with opposite chiralities retrieve at the critical momenta and the corresponding group velocities become zero. Further, the isotropic in-plane Zeeman field leads to rotation of the band structure, as expected, resulting in non-zero group velocities along all directions. Finally, for the sake of completeness, the combined Stark and Zeeman effects are tracked and the results show that the system is an insulator at all fields and the group velocities are altered more than when the individual Stark and Zeeman effects are applied. Our findings may provide interesting physical insights for practical applications in nanoelectronics and spintronics.

On the in-plane electronic thermal conductivity of biased nanosheet β12-borophene

The unique physical and chemical properties of β₁₂-borophene stem from the coexistence of the Dirac and triplet fermions. The metallic phase of β₁₂-borophene transitions to the semiconducting one when it is subjected to a perpendicular electric field or bias voltage. In this work, with the aid of a five-band tight-binding Hamiltonian, the Green's function approach and the Kubo-Greenwood formalism, the electronic thermal conductivity (ETC) of the semiconducting phase of β₁₂-borophene is studied. Two homogeneous (H) and inversion symmetric (IS) models are considered depending on the interaction of the substrate and boron atoms. In addition, due to the anisotropic structure of β₁₂-borophene, the swapping effect of bias poles is addressed. First of all, we find the pristine ETC^IS < ETC^H independent of the temperature. Furthermore, a decrease of 74.45% (80.62%) is observed for ETC^H (ETC^IS) when strong positive bias voltages are applied, while this is 25.2% (47.48%) when applying strong negative bias voltages. Moreover, the shoulder temperature of both models increases (fluctuates) with the positive (negative) bias voltage. Our numerical results pave the way for setting up future experimental thermoelectric devices in order to achieve the highest performance.

Computational insights into structural, electronic and optical characteristics of GeC/C2N van der Waals heterostructures: effects of strain engineering and electric field

Vertical heterostructures from two or more than two two-dimensional materials are recently considered as an effective tool for tuning the electronic properties of materials and for designing future high-performance nanodevices. Here, using first principles calculations, we propose a GeC/C₂N van der Waals heterostructure and investigate its electronic and optical properties. We demonstrate that the intrinsic electronic properties of both GeC and C₂N monolayers are quite preserved in GeC/C₂N HTS owing to the weak forces. At the equilibrium configuration, GeC/C₂N HTS forms the type-II band alignment with an indirect band gap of 0.42 eV, which can be considered to improve the effective separation of electrons and holes. Besides, GeC/C₂N vdW-HTS exhibits strong absorption in both visible and near ultra-violet regions with an intensity of 10⁵ cm⁻¹. The electronic properties of GeC/C₂N HTS can be tuned by applying an electric field and vertical strains. The semiconductor to metal transition can be achieved in GeC/C₂N HTS in the case when the positive electric field of +0.3 V Å⁻¹ or the tensile vertical strain of −0.9 Å is applied. These findings demonstrate that GeC/C₂N HTS can be used to design future high-performance multifunctional devices.

Vertical heterostructures from two or more than two two-dimensional materials are recently considered as an effective tool for tuning the electronic properties of materials and for designing future high-performance nanodevices.

Schottky anomaly and Néel temperature treatment of possible perturbed hydrogenated AA-stacked graphene, SiC, and h-BN bilayers

In this paper, the potential of engineering and manipulating the electronic heat capacity and Pauli susceptibility of pristine and perturbed hydrogenated AA-stacked graphene, SiC (silicon carbide), and h-BN (hexagonal boron nitride) bilayers is studied using a designed transverse Zeeman magnetic field and the dilute charged impurity. The tight-binding Hamiltonian model, the Born approximation and the Green's function method describe the carrier dynamics up to a certain degree. The unperturbed results show that the heat capacity and susceptibility of all bilayers increase with different hydrogenation doping configurations. We also found that the maximum heat capacity and susceptibility relates to the chair-like and table-like configurations. Also, the graphene possesses the highest activity compared to SiC and h-BN lattices due to its zero on-site energies. For the Zeeman magnetic field-induced Schottky anomaly and the Néel temperature corresponding to the maximum electronic heat capacity, EHC_Max., and Pauli spin paramagnetic susceptibility, PSPS_Max., respectively, the pristine EHC_Max. (PSPS_Max.) decreases (increases) with the Zeeman field. On the other hand, the corresponding results for reduced table-like and reduced chair-like lattices illustrate that both EHC_Max. and PSPS_Max. decrease with the Zeeman field, on average. However, under the influence of the dilute charged impurity, the pristine EHC_Max. of graphene (SiC and h-BN) decreases (increases) with impurity concentration for all configurations while the corresponding PSPS_Max. fluctuates (decreases) for the pristine (reduced table-like and reduced chair-like) case. These findings introduce hydrogenated AA-stacked bilayers as versatile candidates for real applications.

Modified tailoring the electronic phase and emergence of midstates in impurity-imbrued armchair graphene nanoribbons

We theoretically address the electronic structure of mono- and simple bi-layer armchair graphene nanoribbons (AGNRs) when they are infected by extrinsic charged dilute impurity. This is done with the aid of the modified tight-binding method considering the edge effects and the Green’s function approach. Also, the interplay of host and guest electrons are studied within the full self-consistent Born approximation. Given that the main basic electronic features can be captured from the electronic density of states (DOS), we focus on the perturbed DOS of lattices corresponding to the different widths. The modified model says that there is no metallic phase due to the edge states. We found that the impurity effects lead to the emergence of midgap states in DOS of both systems so that a semiconductor-to-semimetal phase transition occurs at strong enough impurity concentrations and/or impurity scattering potentials. The intensity of semiconductor-to-semimetal phase transition in monolayer (bilayer) ultra-narrow (realistic) ribbons is sharper than bilayers (monolayers). In both lattices, electron-hole symmetry breaks down as a result of induced-impurity states. The findings of this research would provide a base for future experimental studies and improve the applications of AGNRs in logic semiconductor devices in industry.

Structural and electronic properties of a van der Waals heterostructure based on silicene and gallium selenide: effect of strain and electric field

Combining van der Waals heterostructures by stacking different two-dimensional materials on top of each other layer-by-layer can enhance their desired properties and greatly extend the applications of the parent materials. In this work, by means of first principles calculations, we investigate systematically the structural and electronic properties of six different stacking configurations of a Si/GaSe heterostructure. The effect of biaxial strain and electric field on the electronic properties of the most energetically stable configuration of the Si/GaSe heterostructure has also been discussed. At the equilibrium state, the electronic properties of the Si/GaSe heterostructure in all its stacking configurations are well kept as compared with that of single layers owing to their weak van der Waals interactions. Interestingly, we find that a sizable band gap is opened at the Dirac K point of silicene in the Si/GaSe heterostructure, which could be further controlled by biaxial strain or electric field. These findings open up a possibility for designing silicene-based electronic devices, which exhibit a controllable band gap. Furthermore, the Si/GaSe heterostructure forms an n-type Schottky contact with a small Schottky barrier height of 0.23 eV. A transformation from the n-type Schottky contact to a p-type one, or from the Schottky contact to an ohmic contact may occur in the Si/GaSe heterostructure when strain or an electric field is applied.

Interlayer coupling and electric field tunable electronic properties and Schottky barrier in a graphene/bilayer-GaSe van der Waals heterostructure

In this work, using density functional theory we investigated systematically the electronic properties and Schottky barrier modulation in a multilayer graphene/bilayer-GaSe heterostructure by varying the interlayer spacing and by applying an external electric field.